Challenge 3: copy() and a Companion Object — Solution

data class Book(val title: String, val author: String, val year: Int) {
    companion object {
        fun unknown() = Book("Unknown", "Unknown", 0)
    }
}

fun main() {
    val firstEdition = Book("Kotlin in Action", "Dmitry Jemerov", 2017)
    val secondEdition = firstEdition.copy(year = 2021)

    println(firstEdition)
    println(secondEdition)

    val placeholder = Book.unknown()
    println(placeholder)
}

/*
Output:
Book(title=Kotlin in Action, author=Dmitry Jemerov, year=2017)
Book(title=Kotlin in Action, author=Dmitry Jemerov, year=2021)
Book(title=Unknown, author=Unknown, year=0)

Notes:
- copy(year = 2021) creates a brand-new Book with the same title and
  author as firstEdition, but a different year — firstEdition itself is
  never mutated (all its properties are val).
- Book.unknown() is called directly on the class, exactly like a Java
  static method, but it's implemented as a real function inside the
  class's companion object.
*/